The use of zeolitic materials, and even more generally molecular sieves, in various separations of organic materials is too well known to require extensive comment. Their ability to separate materials based on size differences of the substrates has been found especially useful and has led to a plethora of commercial processes. Although size-based separations are attributed to pores or channels within the crystalline framework of zeolites and molecular sieves, often superimposed on this are charge or van der Waals effects between the surface of the adsorbent and some portion of the substrate molecule.
Silicalites constitute a class of materials which may be viewed as a microporous silica polymorph with the same topology as ZSM-5, i.e., MFl topology. Although synthesis using exceptionally pure reagents can produce an essentially pure silica silicalite, in most usual preparations small amounts of aluminum in the variously employed starting silica sources result in silicalites with typical product silica to alumina (SiO.sub.2 :Al.sub.2 O.sub.3) oxide rations in the range of 200 to 1000. Separations effected with silicalite are based largely on van der Waals effects with the nature of the silicalite surface playing a dominant role.
Silicalites as a class are hydrophobic, yet there remains a residual affinity for water and other polar materials which affect the characteristics of silicalites as adsorbents. One general result of residual hydrophilicity, thought to arise from silanol groups, Si--OH, is substantial tailing of polar materials, for example, alcohols, as adsorbates in chromatographic separations. Clearly, tailing interferes with a chromatographic separation being used either as an analytical tool, because of the difficulty of accurate integration of an asymmetric detector response, or as a purification tool where the tail of a component may extend into the eluate fraction of a different component. Izod et al. recognized this shortcoming, and in U.S. Pat. No. 4,375,568 described the use of F-silicalite (U.S. Pat. No. 4,073,865) in the separation of low molecular weight alcohols, ethylene glycol, and water.
F-silicalite is a silica polymorph containing approximately 0.9% fluorine prior to removal of the template, with properties quite similar to silicalite. It is distinguishable from the latter after calcination in air by its X-ray diffraction pattern, water adsorption properties, and infrared absorption spectrum. Unfortunately, F-silicalite normally is formed as laths, typically of a size 20.times.20.times.150 microns. Such a crystal form is quite unsuitable for high pressure liquid chromatography (HPLC), where the adsorbent preferably is in the form of monodisperse spheres, i.e., spheres of the same size or within a very narrow size range, of size 1 to 20 microns to ensure a high loading of adsorbent without excessive pressure drop. Accordingly, there is a need for a silicalite whose morphology better approximates the ideal of 1-20 micron monodisperse spheres with an exceptional degree of hydrophobicity.
Fatty acids, especially linear, terminal carboxylic acids in the C.sub.10 -C.sub.20 range, are components of many products, especially edible oils, and their measurement is a routine analytical need. Because of the high polarity of carboxylic acids, which leads to their severe tailing on most adsorbents used in chromatography, perhaps the most prevalent analytical technique requires their conversion to an ester, which is far less polar than the precursor carboxylic acid, with subsequent analysis of the resulting ester. Although this conversion is routine, clearly it is desirable to be able to perform analysis on the fatty acids themselves. It occurred to us that extremely hydrophobic silicalites--a type which we refer to as ultrahydrophobic silicalite--would be especially suitable for this type of analysis. Since HPLC is the analytical method of choice, it appeared to us that monodisperse ultrahydrophobic silicalite spheres of 1-20 micron diameter would prove particularly suitable. In fact, this turned out to be the case. Our invention, then, is ultrahydrophobic silicalite of a particular morphology, viz, monodisperse spheres of 1-20 micron diameter. In another aspect, our invention is the separation of fatty acids on ultrahydrophobic silicalite monodisperse spheres in the 1-20 micron diameter size range. Yet another aspect of our invention is the preparation of ultrahydrophobic, monodisperse silicalite spheres of 1-20 micron diameter.